This invention relates to an implantable cardioverting/defibrillating pacemaker and, more particularly, to a pacemaker of this type that is responsive to both an electrically derived heart rate and to a haemodynamic parameter.
An implantable haemodynamic cardioverting/defibrillating pacemaker is disclosed in United States patent application Ser. No. 481,364 to K. A. Collins, filed Feb. 16, 1990, and entitled "An Implantable Automatic and Haemodynamically Responsive Cardioverting/Defibrillating Pacemaker," which is assigned to the assignee of the present invention, and is incorporated herein by reference. The pacemaker of said application Ser. No. 481,364 overcomes many of the problems of false or inappropriate delivery of therapy that arise out of a reliance solely upon the sensing of the electrical activity of the heart as a means of determining the state of cardiac function.
Among the significant problems that face such an implantable device is the problem of increasing the device's battery life. Research has shown that bradycardia support pacing, in a cardioverting/defibrillating pacemaker, can consume between 10 and 30 percent of the total battery life of the device, depending on the pacing voltage used. Minimizing the battery consumption due to bradycardia support pacing will help reduce the risk of premature depletion of the battery of the device. At best, premature depletion forces the recipient of the device to undergo another general anesthetic. At worst, it may result in the death of the patient. Williams et al. in their article "Automatic Implantable Cardioverter-Defibrillator-Related Complications", J.A.C.C., Vol. 15 (2), Page 55A (1990), report that the death rate due to premature battery depletion in such devices was 0.6 percent.
It is well known that under different circumstances greater pacing energies are required. For example, after reversion of ventricular fibrillation, bradycardia support pacing often requires an increased pacing energy. One means of increasing pacing energies during a period of post reversion asystole is disclosed in the U.S. Pat. No. 4,869,252 of Norma L. Gilli, dated Sep. 26, 1989, which patent is assigned to the assignee of the present invention. In the disclosure of said Gilli patent, maximum pacing energies are provided regardless of the state of the heart. The Gilli patent also reveals a method for controlling pulse energy in antitachyarrhythmia and bradycardia support pacing. However, the Gilli patent is limited in that no means are provided for assuring that the capture of the heart is occurring, nor is any means provided therein for determining how low a pacing voltage can safely be used to ensure capture of the heart.
Mirowski et al., in U.S. Pat. Nos. 3,614,955 and 3,942,536, describe systems that sense heart function using the peak of the right ventricular pressure waveform. These devices suffer some obvious disadvantages including, among others, the fact that no haemodynamic parameters are provided to control the pacing voltage used during bradycardia support pacing.
A device disclosed in U.S. Pat. No. 4,774,950, to Cohen, has sought to overcome the shortcomings of common forms of cardioverting/defibrillating pacemakers by relying on the mean right ventricular pressure, mean arterial pressure, mean left atrial pressure, mean left ventricular pressure and/or mean central venous pressure as indicators of haemodynamic compromise. Background material relating to the Cohen invention can be found in an article by Cohen et al., entitled "Haemodynamic Responses to Rapid Pacing: A Model for Tachycardia Differentiation", PACE, Vol. 11, Pages 1522-1528 (1988). The Cohen device also is devoid of disclosure relating to the use of haemodynamic parameters to control the pacing voltage used during bradycardia support pacing.
As indicated earlier, one means of minimizing the energy consumed in bradycardia support pacing, and thereby extending the battery life of the pacemaker, is to reduce the pacing voltage used. In order to do this, the implantable device must have some means of determining whether or not the delivered pacing pulse has stimulated the heart to beat, i.e., determining whether or not the heart has been captured. In a haemodynamic cardioverting/defibrillating pacemaker, the signal from which the haemodynamic parameter is being derived can be used to determine whether or not the pacing pulse has captured the heart. A response to pacing, such as a pulsatile rise in the right ventricular pressure, can be used to confirm capture. The bradycardia support pacing voltage can thereafter be reduced until capture is lost, and then raised again to the lowest effective pacing voltage.
It is, therefore, a primary object of the present invention to provide an improved implantable, battery-operated device for the treatment of a malfunctioning heart that is capable of providing effective bradycardia support pacing at a lower mean energy level than has heretofore been the case in order to provide for increased battery life of the device.
Another object of the present invention is to provide an improved implantable, battery-operated device for the treatment of a malfunctioning heart, which device has sensing modalities for sensing both the heart's electrical function and its haemodynamic function, and which device utilizes sensed haemodynamic function information to enable it reduce the bradycardia support pacing voltage to the lowest effective level.
A further object of the present invention is to provide a two-sensing-modality, cardioverting/defibrillating pacemaking device in which the energy required for bradycardia support pacing can be minimized by sensing evoked ventricular pressure responses to bradycardia support pacing.
Additional objects of the invention include the provision of improved methods (i) for determining a patient's need for bradycardia support pacing; (ii) for optimizing bradycardia support pacing voltages delivered to a patient's heart; and, (iii) for terminating ongoing bradycardia support pacing of a patient's heart when the need for such pacing no longer exists.
Further objects and advantages of this invention will become apparent as the following description proceeds.
______________________________________ SUMMARY OF TERMS ______________________________________ ATP Antitachycardia Pacing. ECG The ECG is, strictly speaking, a gra- phical representation of the electrical activity of the heart. However, the term ECG is used loosely to refer to the electrical activity of heart. The electrical activity of the heart can be sensed either on the surface of the skin, or on or in the heart. L (Left) Used to signify that an acronym refers to the left side of the heart as in: LVFPPA - Left Ventricular Filtered Peak-to-Peak Amplitude (VFPPA); LVP - Left Ventricular Pressure (VP); LVPPF - Left Ventricular Peak Pressure Function (VPPF); etc. R (Right) Used to signify that an acronym refers to the right side of the heart as in: RVFPPA Right Ventricular Filtered Peak- to-Peak Amplitude (VFPPA); RVP - Right Ventricular Pressure (VP); RVPPF - Right Ventricular Peak Pressure Func- tion (VPPF); etc. VF Ventricular Fibrillation. VFPPA Ventricular Filtered Peak-to-Peak Amplitude. VP Ventricular Pressure. VPPF Ventricular Peak Pressure Function. VT Ventricular Tachycardia. ______________________________________